In oil and gas exploration and production operations, bores are drilled from surface to access subsurface hydrocarbon-bearing formations. The bores are lined with bore wall-stabilising metal tubing, generally known as casing or liner, which is cemented in the drilled bore. Bores are typically drilled in sections, with casing being run in to line each bore section as soon as possible following completion of the drilling operation. The cementing operation is generally carried out immediately after the casing has been run into the drilled bore. Typically, cement slurry is circulated from surface through the running string on which the casing is supported, through the casing itself, through an opening in a shoe on the end of the casing, and then up through the annulus between the casing and the wall of the drilled bore.
There are many difficulties associated with achieving a successful cementing operation, for example it is necessary to allow the fluid displaced from the annulus by the cement to pass into the bore, and this may require the provision of complex porting arrangements. Further, achieving an even distribution of cement around the casing is known to be problematic. Further, a conventional cementing operation not only fills the annulus between the casing and the bore wall with cement, but also produces a slug of cement in the end of the bore, which must be drilled out if the bore is to be extended further.
Similar problems are also experienced when cementing expandable tubing, and in cementing casing and liners in “monobore” wells, that is where successive sections of casing or liner are of similar diameter. WO 99/35368 (Shell Internationale Research Maatschappij B. V.) describes a method for drilling and completing a hydrocarbon production well. In one embodiment, a well is lined by successive sections of casing which are expanded in the bore using an expansion mandrel to create a cased bore of substantially constant cross section. Adjacent casing sections overlap, and when the expansion mandrel reaches the overlap the lower casing section further expands the previously expanded upper casing section to create a sealed bond. The document recognises that this will involve increased expansion forces, and it is suggested that the bottom of the upper casing section may be pre-expanded and/or provided with slits or grooves which widen or break open during the expansion process. However, it is noted that the former option would only be available in the first casing section, and only if the first casing section was itself not subject to expansion; subsequent casing sections could not be run through previous cased sections of bore if they had been pre-expanded. Further, it is likely that the latter proposal, that is providing slits or grooves, would weaken the resulting bond and make creation of a sealed bond more difficult. The proposed bore-casing system also overlooks the difficulties involved in expanding a section of previously cemented casing; where there is set cement filling the annulus between the casing and the bore wall, it is likely to be difficult if not impossible to expand the casing.
It is among the objectives of embodiments of the present invention to obviate and mitigate these and other disadvantages of the prior art. It is among further objectives of embodiments of the present invention to provide apparatus and methods suitable for cementing expandable tubing, and in cementing casing and liners in “monobore” wells, that is where successive sections of casing or liner are of similar diameter.